英国医生塞达尼·任格在对离体蛙心进行实验时发现,用不含钙和钾的生理盐水灌注蛙心,蛙心收缩不能维持,用含少量钙和钾的钠盐溶液灌注时,蛙心可持续跳动数小时。这个实验说明钙盐和钾盐( )
Many scientists believe that robots ______ able to talk like humans in 50years.( )
A.wereB.areC.will beD.have been
Why does the earth look blue in space?
Because most of the earth's surface ______ by water.( )
A.coversB.is covered
C.coverD.are covered
Look! The model is walking to us.______ nice dress!( )
A.WhatB.What aC.HowD.How a
What were you doing ______ I knocked at the door?
I was sleeping.( )
A.unlessB.onceC.whenD.while
Mount Xiaolei is not far away from here,but it will still take us a few hours ______ there by bike.( )
A.gettingB.to getC.gotD.get